PHP在类中声明由另一个变量构成的变量

Question

I yesterday discovered strange issue in my class - hope you know some answers. Consider such a class:

class Person {

public $height = 90;
public $weight = $this->height * 0.8;

}

This class returns an error "Parse error: syntax error, unexpected T_VARIABLE" and it seems i can not declare variable in class that is variable itself. Can I only set "static" values to variables in class (i mean static like directly declared like string or int no static like "static $var = 'xyz'";Why is that happening?

Thanks, Kalreg.

Answer 1

Currently, you can use only constant expressions when defining default properties values in PHP. That means you can not use anything that will be evaluated at run-time. Since $this refers to dynamic instance value, it is run-time, obviously, and cannot be used in such definitions.